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The effects of minor elements, i. e., Pb, Bi, and Sb, and Ce on the microstructures and mechanical properties of heavy section dutile cast irons were investigated in this study. The experimental results indicate that degenerate forms of graphite tend to precipitate in ductile iron castings that solidify over an extended period of time. Additions of small amoumts of minor elements, such as 0.005% Pb or 0.003% Bi or 0.008% Sb, result in increased nodule count and improved nodularity. However, substantial graphite deterioration occurs as the addition amounts of those minor elements become excessive, for instance 0.015% Pb, or 0.006% Bi, or 0.016% Sb. The subversive effects of Pb, Bi and Sb can be neutralized by rare earths ( Ce ) addition in appropriate ratios, i.e., Ce/Pb=0.9, Ce/Bi=1.1 and Ce/Sb=0.9.
CHanges in the microstructures of heavy section ductile irons brought about by Pb additions and/or Ce neutralization exert no significant influence on both tensile and yield strengths. The is due to the fact that an improvement in both nodularity and nodule count ( in the case of a small Pb addition or Ce neutralization ) also promotes ferrite formation. These two factors counteract each other in terms of strength. However, the tensile ductility and impact values are substantially increased as the graphite nodularity and nodule count are increased. Similar results were obtained for Bi. With respect to Sb, due to its being a strong pearlite promoter, the tensile strengths tend to increase but the elongation and impact values are greatly reduced as the Sb addition amount is increased. When the harmful effect of Sb was completely neutralized by Ce, the ductility and toughness are increased significaritly, but the strength and hardness are slightly decreased.
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